After a MySQL upgrade, connections to our DB from our Web tier were taking ~30 seconds. It was infuriating.

We weren’t the first to see this:

After three years of running flawlessly, out MySQL database started experiencing problems handling connections. We soon discovered that MySQL by default does a reverse DNS lookup as part of its security process. We discovered that the reverse DNS lookups performed by MySQL were either timing out or were taking a very long time (5-10 seconds). We also discovered that another peice of software- a socket service listening on port 211 began to fail as well. This was a third party product so we are not sure, but the presumption is that it must also use reverse DNS lookups.